Support GitHub Markdown Alerts

[Spenhouet]

Milkdown currently doesn't support Github Alerts, as detailed in the Github Docs.

When I input something like the following:

> [!TIP]
> Lorem Ipsum

GitHub renders this like this:

Tip

Lorem Ipsum

Instead Milkdown renders this:

[Amodh1257]

+1 this. I even have this extension: https://marketplace.visualstudio.com/items?itemName=yahyabatulu.vscode-markdown-alert by Yahya Batulu installed, which works like a charm on regular VSCode markdown, but doesn't work with this extension unfortunately. This feature would be immensely useful, as the most commonly edited markdown documents, are Github READMEs.